Allow attributes on statements

[huonw]

e.g.

#[allow(uppercase_variable)]
let L = [1,2];

#[only_uses(ffi)] // (hypothetical)
unsafe { 
    // ...
}

[thestinger]

Another use case is a clean way of annotating branch metadata on match and if.

[alexcrichton]

This is a change to the language grammar, and could have significant impact on the AST. This may warrant an RFC rather than an issue.

[flaper87]

There's a PR proposing attributes on match arms (#12812), I agree this change and the PR I mentioned should probably have an RFC (perhaps they could share the RFC).

[huonw]

I'll write an RFC for "allow attributes on more places" (covering this and #12812) at some point in the nearish future. I can't do it right now so if someone feels like doing it: the floor is open.

[japaric]

@nick29581 This should be moved to rust-lang/rfcs, it needs an RFC like rust-lang/rfcs#49 did.

[rust-highfive]

This issue has been moved to the RFCs repo: rust-lang/rfcs#427